1. Field of the Invention
The present invention relates to a method of manufacturing a magnetoresistive element that is for use in, for example, a thin-film magnetic head, and has a pair of free layers coupled to a pair of shields.
2. Description of the Related Art
Recently, magnetic disk drives have been improved in areal recording density, and thin-film magnetic heads of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has a structure in which a read head including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head including an induction-type electromagnetic transducer for writing are stacked on a substrate.
Examples of MR elements include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer opposite from the spacer layer. The free layer is a ferromagnetic layer whose direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose direction of magnetization is pinned. The antiferromagnetic layer is a layer that pins the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer. For spin-valve GMR elements, the spacer layer is a nonmagnetic conductive layer. For TMR elements, the spacer layer is a tunnel barrier layer.
Examples of the read head incorporating a GMR element include one having a current-in-plane (CIP) structure in which a current used for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and one having a current-perpendicular-to-plane (CPP) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element.
The read head has a pair of shields with the MR element therebetween. The distance between the pair of shields is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
As one of MR elements that can achieve a reduction in read gap length, there has been proposed an MR element that includes a pair of ferromagnetic layers each functioning as a free layer, and a spacer layer disposed between the pair of ferromagnetic layers (such an MR element is hereinafter referred to as an MR element of three-layer structure), as disclosed in U.S. Patent Application Publication No. 2009/0034133 A1, for example. In the MR element of three-layer structure, when the pair of ferromagnetic layers are subjected to no external magnetic field, they are magnetized in directions that are antiparallel to each other and parallel to the track width direction. The directions of magnetization of the pair of ferromagnetic layers change in response to an external magnetic field.
In the read head incorporating the MR element of three-layer structure, a bias magnetic field is applied to the pair of ferromagnetic layers. The bias magnetic field changes the directions of magnetization of the pair of ferromagnetic layers so that their directions of magnetization each form an angle of approximately 45 degrees with respect to the track width direction. This makes the relative angle between the directions of magnetization of the pair of ferromagnetic layers approximately 90 degrees. When a signal magnetic field sent from the recording medium is applied to the read head, the relative angle between the directions of magnetization of the pair of ferromagnetic layers changes, and consequently, the MR element changes in resistance. With such a read head, it is possible to detect the signal magnetic field by detecting the resistance of the MR element. The read head incorporating the MR element of three-layer structure allows a much greater reduction in read gap length as compared with a read head incorporating a conventional GMR element.
For the MR element of three-layer structure, one of methods for making the directions of magnetization of the pair of ferromagnetic layers antiparallel to each other when no external magnetic field is applied is to establish antiferromagnetic coupling between the pair of ferromagnetic layers via the spacer layer by the RKKY interaction.
Disadvantageously, however, this method imposes limitation on the material and thickness of the spacer layer so as to allow antiferromagnetic coupling between the pair of ferromagnetic layers. In addition, since this method limits the material of the spacer layer to nonmagnetic conductive materials, it is applicable to neither a TMR element which is expected to provide a high output, nor a GMR element of CPP structure of current-confined-path type which is an MR element that is also expected to provide a high output and whose spacer layer includes a portion that allows the passage of currents and a portion that intercepts the passage of currents. The foregoing method further has the disadvantage that, even if it could be possible to make the directions of magnetization of the pair of ferromagnetic layers antiparallel to each other, it is difficult to make them parallel to the track width direction with reliability.
The inventors of this application then conceived providing a pair of shields with an MR stack interposed therebetween, the MR stack including a pair of free layers and a spacer layer interposed between the pair of free layers, so that the directions of magnetization of the pair of free layers in the MR stack are controlled by the pair of shields. According to this technique, the pair of free layers in the MR stack are magnetically coupled to the pair of shields, and are controlled so that their directions of magnetization are antiparallel to each other. Hereinafter, an MR element that is configured to include the foregoing MR stack and the pair of shields will be referred to as a shield-coupling MR element.
In the shield-coupling MR element described above, the lower one of the pair of shields which lies closer to the substrate will be referred to as a first shield, and the upper one will be referred to as a second shield. The lower one of the pair of free layers will be referred to a first free layer, and the upper one will be referred to as a second free layer. In the shield-coupling MR element, the MR stack and the second shield are stacked in this order on the first shield. The MR stack is patterned into a predetermined shape. The shield-coupling MR element has nonmagnetic layers that are disposed on opposite sides of the MR stack in the track width direction, between the first shield and the second shield.
In order to achieve high recording density, it is needed to reduce the read gap length and reduce the track width as well. To reduce the track width of the foregoing shield-coupling MR element, the MR stack needs to be made smaller in dimension in the track width direction.
In the shield-coupling MR element, the magnetic coupling between the second free layer and the second shield is difficult to secure if the MR stack is small in dimension in the track width direction in particular. In order to ensure the magnetic coupling between the second free layer and the second shield, the MR stack may be configured to include a magnetic cap layer that is located above the second free layer and magnetically coupled to the second free layer. Then, a ferromagnetic layer constituting the second shield may be arranged in contact with the top surface of the magnetic cap layer. When manufacturing the shield-coupling MR element of such a configuration, the MR stack and the nonmagnetic layers can be formed by the following lift-off method. In the method, a layered film that is to be patterned into the MR stack later is initially formed on the first shield. Next, a mask to be used for patterning the layered film is formed on the layered film. The layered film is then etched by using the mask. The nonmagnetic layers are then formed with the mask left intact. The mask is then removed. Next, the top surface of the magnetic cap layer is slightly etched to clean the top surface of the magnetic cap layer.
The mask to be used in the foregoing method has an undercut shape for easy removal later. An example of the method for forming an MR element by the lift-off technique using an undercut mask is disclosed in JP-A-2007-234646, for example.
The inventors of this application actually formed MR stacks and nonmagnetic layers by the foregoing method. As a result, the following problem was found to occur in association with the method. That is, with such a method, the undercut mask makes the shape of the top surface of the magnetic cap layer, which is the uppermost layer of the MR stack, a convex shape with both side portions lower in level than the center portion after the etching of the layered film. When the mask is removed and the top surface of the magnetic cap layer is cleaned, the top surface of the magnetic cap layer becomes more convex. If the top surface of the magnetic cap layer has a convex shape, it follows that the thickness of the magnetic cap layer varies according to the position within the area surrounded by the outer edges of the MR stack when the MR stack is seen from above. If the magnetic cap layer has such uneven thickness in the foregoing area, the stack consisting of the magnetic cap layer and the ferromagnetic layer that is disposed thereon and constitutes the second shield (hereinafter, referred to as the magnetic stack) also has uneven thickness in that area.
The exchange coupling magnetic field that causes magnetic coupling between the second free layer and the second shield depends on the thickness of the magnetic stack in the foregoing area. More specifically, the thicker the magnetic stack, the smaller the exchange coupling magnetic field. The smaller the exchange coupling magnetic field, the higher the shielding capability or flux-absorbing capability of the magnetic stack, but the lower the capability of controlling the direction of magnetization of the second free layer. It is therefore necessary to adjust the exchange coupling magnetic field to an appropriate value. If, as described above, the magnetic stack has uneven thickness in the foregoing area, the exchange coupling magnetic field becomes uneven in the area. This consequently makes the magnetic stack unstable both in its shielding capability and its capability of controlling the direction of magnetization of the second free layer. In such a case, it also becomes difficult to control the average exchange coupling magnetic field in each single MR element, so that characteristic variations among a plurality of MR elements increase. If the MR stack is reduced in dimension in the track width direction, in particular, the portions of the top surface of the magnetic cap layer that are lower in level than the center portion increase in proportion. This makes the foregoing problem even more significant.
U.S. Pat. No. 6,669,983 describes a method of forming an MR element without using the lift-off technique. In the method, a multilayer film that is to be patterned into the MR stack later is initially formed on a lower electrode film that also functions as a magnetic shield film. Next, the multilayer film is etched into the MR stack by using a photoresist pattern as a mask. An insulating film is then formed over the entire surface with the photoresist pattern left intact. Next, the insulating film is polished by chemical mechanical polishing (CMP) until only a small thickness of the photoresist pattern remains on the MR stack. The remaining photoresist pattern is then removed with a solvent. Next, an upper electrode film that also functions as a magnetic shield film is formed over the insulating film and the MR stack. Such a method has the problem of high manufacturing cost since the CMP process is needed. In addition, this method includes the removal of the photoresist pattern, which tends to produce resist residues on the top surface of the MR stack. Removing the resist residues by ashing or the like causes the problem that the MR stack becomes uneven in its top surface.